1. Technical Field
The present invention relates to a flexible substrate and electronic equipment.
2. Related Art
As the resolution of a display panel built in small-sized electronic equipment increases, the need increases for transmitting signals at high frequencies among circuits included in the electronic equipment. There arises the problem of signal reflection among a circuit that transfers high-frequency signals to transmit the signals, a transmission channel that transmits high-frequency signals, and a circuit that receives high-frequency signals. The signal reflection is caused by impedance differences among the circuits and transmission channel, and causes signal degradation and unnecessary radiation.
Japanese Unexamined Patent Application Publication No. 9-83233 describes a portable radio terminal with its sensitivity enhanced by cutting unnecessary radiation in transmitting high-frequency signals. The portable radio terminal shown in FIG. 8A of 9-83233 has a ground layer on the surface of a flexible substrate coupling a high-frequency circuit and a base band circuit, in order to cut unnecessary radiation with the ground layer acting as a shield. Accordingly, while the sensitivity of the portable radio terminal increases, impedance matching is not achieved among the input-output impedance of the high-frequency circuit, the characteristic impedance of wirings on the flexible substrate, and the input-output impedance of the base band circuit. Therefore, it is difficult to transmit signals at higher frequencies. Impedance matching among various media coupling the high-frequency circuit, the flexible substrate, and the base band circuit in the circuitry described in 9-83233 could largely increase design cost, which is unpractical.
Japanese Unexamined Patent Application Publication No. 2002-354053 describes a signal transmission cable fitted with impedance parts on both sides of its differential signal transmission channel, which transmits differential signals. This structure aims to improve the characteristics of differential signals. However, in order to prevent signal reflection between a circuit for transmitting differential signals and a circuit for receiving differential signals, it is necessary to get the output impedance of the circuit transmitting differential signals and the input impedance of the circuit receiving differential signals matched to the terminal impedance of the signal transmission cable described in 2002-354053. This impedance matching makes the design of the circuits for transmitting and receiving differential signals complicated and keeps cost high, resulting in cost increases even with economical cables. Moreover, extra impedance parts added to the transmission channel can increase the amount of power consumption. Therefore, such a cable is not preferably applicable to portable electronic equipment, in particular.
Japanese Unexamined Patent Application Publication No. 5-27697 below also describes a method for cutting unnecessary radiation by means of impedance matching. However, in order to cut signal reflection at both ends of a transmission channel through which high-frequency signals are transmitted, the structure disclosed by 5-27697 requires impedance matching among circuits coupled to the both ends of the transmission channel, which keeps the cost of design high.
In consideration of the above-mentioned technical issues, the present invention is intended to provide a flexible substrate that is highly cost effective and capable of cutting signal reflection in transmitting high-frequency signals and transferring serial data at high speed with little signal degradation, and also to provide electronic equipment including the flexible substrate.